MAGNETIC PHASE-DIAGRAM AND VORTEX PINNING IN YBA2CU3OY SINGLE-CRYSTALS

Magnetization measurements in YBa2Cu3Oy single crystals are performed as a function of the temperature T and the magnetic field H parallel t o the c-axis. We find that the critical current density J(c) shows a s harp peak just below the vortex lattice melting line H-m(T); the peak of J(c)(T) is enhanced when H-m(T) agrees with the irreversibility lin e H-irr(T). J(c) shows large T dependence below the peak, following th e power law dependence of J(c) proportional to (1-t)(n) with n similar or equal to 5.5. The temperature dependence of J(c) is well described by the collective pinning theory using the elemental pinning force fo r small defects.